Semiconductor chip manufacturers have increasingly sought to improve yields in their production processes. Key to this effort is the identification of particulate contamination and wafer defects during wafer processing. One tool available to identify contamination and defects is a laser imaging system that utilizes confocal laser scanning microscopy techniques, also known as a confocal microscope. Such a microscope includes a laser light source that emits a laser beam focussed on a pinhole aperture in a focal plane of an objective lens. A beam scanner receives the beam after it exits the pinhole aperture and, using moving reflective elements, scans the beam through the objective lens and across a surface to be imaged.
The objective lens has a second focal plane, on the side of the objective lens opposite the pinhole aperture, in which an image of the pinhole aperture is formed. A photodetector in the return path of laser light reflected from the surface generates an output signal proportional to the intensity of laser light reflected from the object and back through the lens, the beam scanner, and the pinhole aperture. For a given point on the surface, the reflected intensity, and therefore the output signal from the photodetector, is highest when the surface lies in the second focal plane of the lens. This is because the objective lens focusses the reflected image of the pinhole aperture back through the pinhole aperture to the photodetector. In contrast, when the surface does not lie in the second focal plane, the image of the pinhole aperture is out of focus (i.e., the diameter of the image on the surface is much larger than the aperture) so that most of the reflected image does not return through the pinhole aperture.
To obtain an image of a target surface, the target surface is scanned in a number of X-Y planes located along a Z axis generally normal to the target surface. In each scan, the photodetector provides indications of the intensities of the reflected laser light from a number of points on the surface. That is, the objective lens is positioned at each location on the Z axis and the laser beam is scanned across the surface to generate a number of signals, each of the signals representing an intensity of light reflected through the objective lens from a given point on the surface. The group of signals provided by an X-Y scan from a single location of the objective lens on the Z axis is called a "slice" of intensity data. Slices taken from the various locations on the Z axis overlap to form a three-dimensional set of reflected intensity data.
The overlapping slices of data create a column of data values for each point on the surface, each data value representing a reflected intensity of light from that point from a particular Z location. For each such column, data values are compared to determine the location on the Z axis that resulted in a maximum reflected intensity. Because the intensity of the reflected light from a particular point is greatest when that point on the surface is coincident with the focal plane of the objective lens, the location of the objective lens on the Z axis that corresponds to the maximum reflected intensity gives an indication of the Z coordinate of that point on the surface. In this way, the X, Y, and Z Cartesian coordinates are determined for each point on the surface. An image of the surface may then be generated from this information.
For a more detailed description of such a laser imaging system, see the co-pending application entitled "Laser Imaging System For Inspection and Analysis of Sub-Micron Particles," the content of which is incorporated herein by reference.
Confocal microscopes require the user to input sample-specific data before an image of the target sample can be obtained. For example, the user might have to specify the optimal photodetector gain for measuring laser-light reflected from the target surface, the offset in the Z-direction from which the first X-Y scan will begin, the range to be covered in the Z direction, and the optimal number of "slices" taken along the Z axis to form the three-dimensional set of reflected intensity data.
Assume, for example, that time constraints limit the number of slices to fifty. If the sample is very flat, resolution should be maximized by taking the fifty slices over a relatively short Z range, whereas if the surface of the sample is relatively rough (i.e., the surface features are "tall"), the distance between adjacent slices should be optimized so that the total Z range captures the low and high regions of the surface. And, whatever the surface texture, the scan range should be optimized so that the scan precisely covers the Z range of the surface features so that almost all of the fifty slices provide surface data.
Unfortunately, the process of setting up a confocal microscope for different types of target samples can be difficult and tedious. There is therefore a need to obtain a surface image using a confocal microscope without requiring the user to manually optimize scan parameters.